The invention relates in general to the use of high resolution microscopy probe stations, and particularly to methods and system for probing with electrical test signals on integrated circuit (IC) specimens using a scanning electron microscope (SEM) positioned for observing the surface indicia of the specimen identifying the electrically conductive terminals for the positioning of the probes.
Presently, probe stations typically employ optical microscopes. Although the diameters of wafers are getting larger, the structures constructed on and in those wafers are getting smaller. In the past several decades, the industry has driven the size of these structures from large sizes on the order of hundredths of an inch to small fractions of micrometers today. Until recently, most structures could be observed by normal high magnification light microscopes and probed. However, modern structures have now achieved a size that no longer allows viewing with standard light microscopes. With the industry integrated circuit design rules driving towards 0.18 micron features and smaller, most advanced optical light microscopes cannot be relied upon to accurately identify the electrically conductive terminals from the conductive path indicia of the surface of the integrated circuit specimens under test. Additionally, when viewing very small features on a specimen, the optical microscope lens often must be positioned so close to the specimen that it may interfere with the test probes.
Another approach is necessary in addition to optical microscopy if the industry is to continue to probe these structures, which is surely needed. It would be desirable therefore to provide a probe station which can visualize and probe features not typically visible under even the most advanced light microscope, that can be used in conjunction with electron optics while maintaining the features typically found on optical microscope probe stations.
Briefly summarized, the present invention relates to a method and system for probing with electrical test signals a specimen using high resolution microscopy, such as a scanning electron microscope (SEM) or a Focus Ion Beam (FIB) system, positioned for observing a surface of the specimen to identify locations of electrically conductive terminals on the specimen. In a preferred form, a carrier is provided for supporting the specimen in relation to the scanning electron microscope while a controller, such as a computer, acquires an image identifying conductive path indicia of the surface of the specimen from the scanning electron microscope. The carrier may be anyone of a number of items known to one of ordinary skill in the art, such as a chuck (e.g., ambient, thermal, triaxial, etc.), a probe card adapter and probe card, a socket stage adapter, etc.
Motorized manipulators can be automatically controlled by the computer, or manually by the operator using a joystick or the like, to precisely position associated probes on or near the surface of the specimen for acquiring and conveying electrical test signals inside a vacuum chamber inner enclosure which houses at least a portion of the scanning electron microscope, the carrier, the motorized manipulators and probes for analyzing the specimen in a vacuum. A feedthrough or electrical connector mounted to the vacuum chamber allows for the computer to be electrically interconnected to the motorized manipulators and their associated probes in the sealed enclosure and can provide access to the internal vacuum chamber for additional wiring and conduits. The computer communicates with the motorized manipulators for positioning the probes thereof, and for acquiring and applying electrical test signals from and/or to the terminals on the specimen using the image acquired by the computer to identify the electrically conductive terminals from the conductive path indicia of the surface of the specimen observed with the scanning electron microscope.
The computer includes a display which shows a viewer an enlarged view of the surface of the specimen being probed. A cursor indicates the selected location or test site on the specimen at which test signals are transferred to and from the probe. In this manner, an operator can change selected test locations via on-screen manipulation of the cursor, as by a mouse or other computer interface control. Moving the cursor causes the relative position between the probe and the specimen surface to shift under software control so that the probe is oriented at the selected test site. To this end, the software is programed to operate actuators of the probe assemblies and/or the carrier on which the specimen is affixed for precision shifting thereof to position the probe at the selected test site. Accordingly, with a mouse, an operator can click on the cursor, and drag it across the screen to the desired conductive path indicia location or terminal they desire to test.
To improve low current testing accuracy, the preferred probing system is highly flexible in allowing for different guarding and/or shielding schemes to be employed throughout substantially every level of its operating components. For example, the probe station housing can be separated into two electrically isolated outer and inner portions each having conductive walls so that the inner portion can be driven to the same potential as the signal applied to the specimen to assist in isolating the testing area from noise and other environmental interference and the outer portion can be grounded to reduce the risk of electrical shock to probe station users. The probes and chuck can be wired in a similar configuration to further isolate the testing area from noise and interference. Further, locations of the electrical interconnects can be selected to minimize lengths of wiring runs from the chamber walls to the operating components, e.g., probe and chuck and their actuators or motors.
To compensate for the sources of heat and radiation of heat within the vacuum chamber, the drive mechanisms of the system are constructed of heat insulating materials having low coefficients of thermal expansion to insulate components of the drive mechanisms from heat and unwanted movement or drift caused by thermal expansion, and have radiation shields for deflected heat or energy from the motors of the drive systems towards the housing walls which are better equipped to handle the buildup of heat due to their proximity to the outer atmoshphere.
In other aspects, the probes can include extended cladding to minimize the amount of unwanted insulator charging. A touchdown sensing mechanism can be utilized to reduce the risk of damage to the specimen caused by excessive force applied thereto by probe engagement. The duty cycle of the high resolution microscope is preferably reduced as by a shuttering system. In this way, damage done to the DUT via the beam of the microscope is minimized.